Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1-xKxFe2As2 superconductors

TL;DR

This study uses atomic resolution imaging to reveal nanometer-scale amorphous layers, oxide crystallites, and oxygen enrichment at grain boundaries in Sr0.6K0.4Fe2As2 superconductors, explaining their low critical current density.

Contribution

It provides detailed atomic-scale insights into the structure and composition of grain boundaries in polycrystalline Sr1-xKxFe2As2 superconductors, highlighting features previously unobserved.

Findings

Presence of amorphous layers larger than the coherence length at grain boundaries

Nanometer-scale oxide crystallites found between amorphous layers

Significant oxygen enrichment at grain boundaries

Abstract

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.